Semiconductor device

ABSTRACT

Provided is a semiconductor device capable of reducing a temperature-dependent variation of a current sense ratio and accurately detecting current In the semiconductor device, at least one of an impurity concentration and a thickness of each semiconductor layer is adjusted such that a value calculated by a following equation is less than a predetermined value: 
     
       
         
           
             
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                     n 
                   
                    
                   
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                         R 
                         Mi 
                       
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                         k 
                         Mi 
                       
                     
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                 - 
                 
                   
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                    
                   
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                         R 
                         Si 
                       
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                     R 
                     Mi 
                   
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             where a temperature-dependent resistance changing rate of an i-th semiconductor layer (i=1 to n) of the main element domain is R Mi ; a resistance ratio of the i-th semiconductor layer of the main element domain relative to the entire main element domain is k Mi ; a temperature-dependent resistance changing rate of the i-th semiconductor layer of the sense element domain is R Si ; and a resistance ratio of the i-th semiconductor layer of the sense element domain to the entire sense element domain is k Si .

TECHNICAL FIELD

The present invention relates to a semiconductor device having currentsensing function.

BACKGROUND ART

Patent document 1 discloses an exemplary semiconductor device having acurrent sensing function. Patent document 1 includes a main elementdomain and a sense element domain connected in parallel with the mainelement domain. A plurality of main elements is formed in the mainelement domain, and a plurality of sense elements is formed in the senseelement domain. Current flowing through the semiconductor device isdivided into a main current flowing through the main element domain anda sense current flowing through the sense element domain. A main currentvalue is a value corresponding to the number of main elements, and asense current value is a value corresponding to the number of senseelements. Accordingly, a current sense ratio obtained by dividing themain current value by the sense current value is constant. Thus, if thevalue of the sense current flowing through the sense element domain ismeasured, the main current value can be calculated from the measuredsense current value and the current sense ratio.

CITATION LIST Patent Literature

Patent Document 1: Japanese Patent Application Publication No.H10-107282

SUMMARY OF INVENTION Technical Problem

In such a semiconductor device, each of the main element domain and thesense element domain includes a semiconductor region in which aplurality of semiconductor layers is laminated. When the semiconductordevice turns on, current flows in a lamination direction of thesemiconductor layers in the semiconductor region of each of the mainelement domain and the sense element domain. For this reason, theelectric resistance of the main element domain and the electricresistance of the sense element domain are determined by the electricresistance of each semiconductor layer. The semiconductor region of themain element domain and the semiconductor region of the sense elementdomain typically have the same configuration. Specifically, thethicknesses of a semiconductor layer is set to be equal to that of acorresponding semiconductor layer, and the impurity concentration of thesemiconductor layer is set to be equal to that of the correspondingsemiconductor layer. Accordingly, even if the temperature of thesemiconductor device changes, when the temperature of each semiconductorlayer of the main element domain and the temperature of eachsemiconductor layer of the sense element domain change in the same way,the electric resistance of each semiconductor layer of the main elementdomain and the electric resistance of each semiconductor layer of thesense element domain are to change in the same way and the current senseratio is to be maintained at a certain value.

However, because the number of main elements formed in the main elementdomain is greatly different from the number of sense elements formed inthe sense element domain, the size of the main element domain is greatlydifferent from the size of the sense element domain. As a result, thecurrent sense ratio changes depending on the temperature of thesemiconductor device.

Specifically, in the main elements disposed along a boundary between themain element domain and the sense element domain, current flows from theoutside of the main element domain, whereas in the sense elementsdisposed along the boundary, current flows from the outside of the senseelement domain. In the main element domain, the number of main elementsdisposed along the boundary is smaller than the number of main elementsdisposed within the main element domain apart from the boundary.Accordingly, in the main element domain, the effect of current flowingfrom the outside of the main element domain is small. As a result, inthe main element domain, there is no need to take into consideration theeffect of current flowing obliquely with respect to the laminationdirection of the semiconductor layers, and thus it can be consideredthat current flows substantially in parallel with the laminationdirection of semiconductor layers.

On the other hand, in the sense element domain, the number of senseelements disposed along the boundary is greater than the number of senseelements disposed in the sense element domain apart from the boundary.Accordingly, in the sense element domain, the effect of current flowingfrom the outside of the sense element domain is large. As a result, inthe sense element domain, it is necessary to take into consideration theeffect of current flowing obliquely with respect to the laminationdirection of the semiconductor layers. When current flows obliquely withrespect to the lamination direction of the semiconductor layers, aregion (a current path length, a passage section, or the like) in whichcurrent flows through the semiconductor layers changes, so thatresistance values of the semiconductor layers change.

This results in a difference between a resistance ratio of eachsemiconductor layer to the entire electric resistance of the mainelement domain and a resistance ratio of each semiconductor layer to theentire electric resistance of the sense clement domain. In general,temperature-dependent resistance changing rates of each semiconductorlayer are different from each other. Thus, when the resistance ratio ofeach semiconductor layer of the main element domain is different fromthe resistance ratio of each semiconductor layer of the sense elementdomain, the temperature-dependent resistance changing rate of the entiremain element domain is different from the temperature-dependentresistance changing rate of the entire sense element domain.Accordingly, even when the temperature of each semiconductor layer ofthe main element domain and the temperature of each semiconductor layerof the sense element domain change in the same way, the electricresistance of the main element domain and the electric resistance of thesense element domain do not change in the same way and the current senseratio changes.

The present invention aims to provide a semiconductor device capable ofreducing a temperature-dependent variation of a current sense ratio anddetecting current with high accuracy.

Solution to Technical Problem

Referring to FIG. 1, a semiconductor device according to an exemplaryaspect of the present invention includes a main element domain 4 and asense element domain 6 disposed adjacent to the main element domain 4.Each of the main element domain 4 and the sense element domain 6includes a semiconductor region in which semiconductor layers from afirst semiconductor layer to an n-th semiconductor layer are laminatedin order. When the semiconductor device turns on, current flows in alamination direction of the semiconductor layers in each of the mainelement domain 4 and the sense element domain 6. Assume herein that atemperature-dependent resistance changing rate of an i-th semiconductorlayer (i=1 to n) of the main element domain 4 is R_(Mi); a resistanceratio of the i-th semiconductor layer of the main element domain 4relative to the entire main element domain 4 is k_(Mi); atemperature-dependent resistance changing rate of the i-th semiconductorlayer of the sense element domain 6 is R_(Si); and a resistance ratio ofthe i-th semiconductor layer of the sense element domain 6 to the entiresense element domain 6 is k_(Si). In this case, in at least one of thesemiconductor layers from the first semiconductor layer to the n-thsemiconductor layer of each of the main element domain 4 and the senseelement domain 6, at least one of an impurity concentration and athickness of the at least one of the semiconductor layers of the mainelement domain 4 is different from that of the at least one of thesemiconductor layers of the sense element domain 6 such that a valuecalculated by a following equation is less than a predetermined value:

$\begin{matrix}{\left\lbrack {{\sum\limits_{i = 1}^{n}\left( {R_{Mi} \times k_{Mi}} \right)} - {\sum\limits_{i = 1}^{n}\left( {R_{Si} \times k_{Si}} \right)}} \right\rbrack/{\sum\limits_{i = 1}^{n}\left( {R_{Mi} \times k_{Mi}} \right)}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the semiconductor device described above, the temperature-dependentresistance changing rate (R_(Mi), R_(Si)) of each semiconductor layerand the resistance ratio of each semiconductor layer (k_(Mi), k_(Si))relative to the entire semiconductor layers are used. Herein, thetemperature-dependent resistance changing rate is determined by theimpurity concentration or the like of each semiconductor layer and doesnot change depending on the current flowing direction. On the otherhand, the resistance ratio of each semiconductor layer changes dependingon the direction of current flowing through the semiconductor region. Inother words, when current flows through the semiconductor region inparallel with the lamination direction, the resistance ratio of eachsemiconductor layer corresponds to the thickness in the laminationdirection of each semiconductor layer. Meanwhile, when current flowsthrough the semiconductor region obliquely with respect to thelamination direction, the resistance ratio of each semiconductor layerdoes not correspond to the thickness in the lamination direction of thesemiconductor layers. Accordingly, the use of the resistance ratio(k_(Mi), k_(Si)) of each semiconductor layer makes it possible to takeinto consideration the effect of current flowing through thesemiconductor region obliquely with respect to the lamination direction.

In the semiconductor device described above, the temperature-dependentresistance changing rate (R_(Mi) or R_(Si)) of each semiconductor layeris multiplied by the resistance ratio (k_(Mi) or k_(Si)) of eachsemiconductor layer for each of the main element domain 4 and the senseelement domain 6, to thereby calculate the sum of these multiplicationresults. Specifically, calculations are performed to obtain a value forevaluating the temperature-dependent resistance changing rate of theentire main element domain and a value for evaluating thetemperature-dependent resistance changing rate of the entire senseelement domain. Further, at least one of the impurity concentration andthe thickness of at least one of the first to n-th semiconductor layersof the main element domain 4 is set to be different from that of atleast one of the semiconductor layers of the sense element domain 6 sothat the difference between the sum of the multiplication results forthe main element domain 4 and the sum of the multiplication results forthe sense element domain 6 can be reduced. Accordingly, the differencebetween the temperature-dependent resistance changing rate of the entiremain element domain 4 and the temperature-dependent resistance changingrate of the sense element domain 6 is reduced. As a result, atemperature-dependent variation of the current sense ratio can bereduced.

Examples of the semiconductor device described above includesemiconductor devices for power use, such as a MOSFET and an IGBT, butare not limited thereto. Any semiconductor device may be used as long asit includes a main element domain and a sense element domain.Furthermore, the semiconductor layers may be laminated in the verticaldirection or lateral direction.

Semiconductor layers to be calculated in the above equation may beappropriately selected from among a plurality of semiconductor layerseach serving as a current path when the semiconductor device turns on.For example, only semiconductor layers having a large resistancecomponent may be selected from among the semiconductor layers disposedon a current path. Alternatively, all semiconductor layers disposed onthe current path may be selected.

In an exemplary configuration of the semiconductor device describedabove, each of the main element domain and the sense element domain mayinclude a first semiconductor layer, a second semiconductor layerlaminated on the first semiconductor layer, and a third semiconductorlayer laminated on the second semiconductor layer. The impurityconcentration of the second semiconductor layer may be set to be lowerthan that of the first semiconductor layer. When the semiconductordevice turns on, a channel is formed in the third semiconductor layer ofeach of the main element domain and the sense element domain, whichallows current to flow from the second semiconductor layer to the thirdsemiconductor layer or from the third semiconductor layer to the secondsemiconductor layer.

In the semiconductor device described above, the impurity concentrationof the first semiconductor layer is different from the impurityconcentration of the second semiconductor layer. Accordingly, thetemperature-dependent resistance changing rate of the firstsemiconductor layer is different from the temperature-dependentresistance changing rate of the second semiconductor layer. As a result,a temperature-dependent variation of the current sense ratio can bereduced by adjusting the thickness or the like of each of the firstsemiconductor layer and the second semiconductor layer.

In the semiconductor device according to an exemplary aspect of thepresent invention, the impurity concentration of the third semiconductorlayer of the sense element domain can be set to be lower than that ofthe third semiconductor layer of the main element domain, for example.The temperature-dependent resistance changing rate of the thirdsemiconductor layer changes with a change of the impurity concentrationof the third semiconductor layer. Accordingly, the impurityconcentration of the third semiconductor layer of the sense elementdomain is set to a value different from the impurity concentration ofthe third semiconductor layer of the main element domain, thereby makingit possible to reduce the value calculated by the above equation. Thisleads to a reduction in the temperature-dependent variation of thecurrent sense ratio.

In the semiconductor device according to an exemplary aspect of thepresent invention, the thickness of the third semiconductor layer of thesense element domain can be made smaller than that of the thirdsemiconductor layer of the main element domain. The resistance ratio ofthe third semiconductor layer can be changed by changing the thicknessof the third semiconductor layer. Accordingly, the thickness of thethird semiconductor layer of the sense element domain is set to a valuedifferent from the thickness of the third semiconductor layer of themain element domain, thereby making it possible to reduce the valuecalculated by the above equation. This leads to a reduction in thetemperature-dependent variation of the current sense ratio.

In the semiconductor device according to an aspect of the presentinvention, the thickness of the first semiconductor layer of the senseelement domain may be set to be larger than the thickness of the firstsemiconductor layer of the main element domain. Also in thisconfiguration, a temperature-dependent variation of the current senseratio can be reduced.

In an exemplary configuration of the semiconductor device describedabove, the thickness of the second semiconductor layer of the senseelement domain may be made smaller than that of the second semiconductorlayer of the main element domain. Also in such a configuration, atemperature-dependent variation of the current sense ratio can bereduced.

In the semiconductor device described above, it is preferable to form atrench between the main element domain and the sense element domain. Theformation of a trench between the main element domain and the senseelement domain prevents current from flowing obliquely through thesemiconductor region. Consequently, a temperature-dependent variation ofthe current sense ratio can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram for explaining a configuration of asemiconductor device according to an exemplary aspect of the presentinvention.

FIG. 2 is a sectional view of a main part of a semiconductor device ofan embodiment.

FIG. 3 is a sectional view of a MOSFET of the semiconductor device of anembodiment.

FIG. 4 is a schematic diagram showing a flow of a main current I_(m)flowing through a main element domain and a flow of a sense currentI_(s) flowing through a sense element domain.

FIG. 5 is a flow chart showing a procedure for determining an impurityconcentration and a thickness of each semiconductor layer of each of themain element domain and the sense element domain.

FIG. 6 is a graph showing an example of temperature-dependent resistancechanging rate of each semiconductor layer.

FIG. 7 is a table showing an example of calculating a resistance ratioof each semiconductor layer of the main element domain and a resistanceratio of each semiconductor layer of the sense element domain.

FIG. 8 shows a simulation model of the main element domain.

FIG. 9 shows a simulation model of the sense element domain.

FIG. 10 is a graph showing another example of the temperature-dependentresistance changing rate of each semiconductor layer.

FIG. 11 is a table showing another calculation example of a resistanceratio of each semiconductor layer of the main element domain and aresistance ratio of each semiconductor layer of the sense elementdomain.

FIG. 12 is a schematic view showing an example in which a trench isformed between the main element domain and the sense element domain.

FIG. 13 is a schematic view showing another example in which a trench isformed between the main element domain and the sense element domain.

FIG. 14 is a schematic view showing still another example in which atrench is formed between the main element domain and the sense elementdomain.

FIG. 15 shows an embodiment in which the thicknesses of a body regionand a drift region are changed in each of the main element domain andthe sense element domain.

FIG. 16 shows an embodiment in which the thickness of the drift regionis changed in each of the main element domain and the sense elementdomain.

FIG. 17 shows an embodiment in which the thickness of a semiconductorsubstrate is changed in each of the main element domain and the senseelement domain.

DESCRIPTION OF EMBODIMENTS

First, the main features of an embodiment will be described in detailbelow.

(First mode) A semiconductor device according to a first mode is avertical semiconductor device in which current flows in the verticaldirection (thickness direction) of a semiconductor substrate.

(Second mode) A semiconductor device according to a second mode is asemiconductor device having the following configuration. That is, aplurality of switching elements is formed in each of a main elementdomain and a sense element domain. Each switching element includes afirst region having a first conductive type, a body region having asecond conductive type, a drift region having the first conductive type,and a gate electrode. The first region faces a first surface of asemiconductor region. The body region faces the first surface of thesemiconductor region and covers the first region. The drift region isseparated from the first region by the body region. The gate electrodefaces the body region in a range where the first region is separatedfrom the drift region, with an insulating film interposed therebetween.

(Embodiment) FIG. 2 shows a sectional view of a main part of asemiconductor device of an embodiment. As shown in FIG. 2, thesemiconductor device includes a semiconductor region 100 in which aplurality of semiconductor layers is laminated. The semiconductor region100 is partitioned into a main element domain 1 and a sense elementdomain 2. In the main element domain 1, a large number of verticalfield-effect transistors (hereinafter, referred to as “MOSFETs”) isformed. Main current flows through the MOSFETs in the main elementdomain 1. A plurality of vertical MOSFETs is formed in the sense elementdomain 2. Sense current flows through the MOSFETs in the sense elementdomain 2. The number of MOSFETs formed within the main element domain 1is much greater than the number of MOSFETs formed within the senseelement domain 2. Accordingly, the size of the main element domain 1 isgreatly different from the size of the sense element domain 2.

A drain electrode 10 is formed on the back surface of the semiconductorregion 100. The drain electrode 10 is made of aluminum. The drainelectrode 10 is shared by the MOSFETs formed in the main element domain1 and the MOSFETs formed in the sense element domain 2.

A main source electrode 81 formed on the main element domain 1 and asense source electrode 82 formed on the sense element domain 2 aredisposed on the surface of the semiconductor region 100. The main sourceelectrode 81 and the sense source electrode 82 are each made ofaluminum. The main source electrode 81 and the sense source electrode 82are electrically insulated by an insulating region 83. In the insulatingregion 83, a source region 61 and a body contact region 62, which willbe described later, are not formed.

The sense source electrode 82 is provided with a sense pad 84. The sensepad 84 is connected to a current detection circuit (not shown) via analuminum wiring line. The main source electrode 81 is provided with asource pad 85. The source pad 85 is connected to a load (for example, amotor) via an aluminum'wiring line. The main source electrode 81 isprovided with a Kelvin pad 86. The Kelvin pad 86 is connected to thecurrent detection circuit via an aluminum wiring line.

Next, the MOSFETs formed in each of the main element domain 1 and thesense element domain 2 will be described. The MOSFETs formed in the mainelement domain 1 have the same configuration as that of the MOSFETsformed in the sense element domain 2. Accordingly, the configuration ofa MOSFET formed in the main element domain 1 will be described by way ofexample.

FIG. 3 shows a sectional view of a MOSFET formed in the main elementdomain 1. As shown in FIG. 3, an n⁺-type semiconductor substrate (drainregion) 20 is formed on the drain electrode 10. The semiconductorsubstrate 20 is a monocrystalline silicon substrate. An n⁻¹-type driftregion 30 is formed on the n+-type semiconductor substrate 20. Theimpurity concentration of the drill region 30 is set to be lower thanthe impurity concentration of the semiconductor substrate 20. A p⁻-typebody region 40 is formed on the n⁻-type drift region 30. An n⁺-typesource region 61 and a p⁺-type body contact region 62 are formed on thep⁻-type body region 40. A trench gate electrode 50 is formed so as topenetrate the n⁺-type source region 61 and the p⁻-type body region 40.The trench gate electrode 50 reaches the drift region 30. Both sides ofthe trench gate electrode 50 face the source region 61 and the bodyregion 40 through a gate insulating film 51. The trench gate electrode50 of each MOSFET within the main element domain 1 and the trench gateelectrode 50 of each MOSFET within the sense element domain 2 areconnected to a common drive circuit (not shown), and turn on/off at thesame timing.

A main source electrode 81 is formed on the surface of each of then⁺-type source region 61 and the p⁺-type body contact region 62. Themain source electrode 81 is electrically insulated from the trench gateelectrode 50 by an insulating film 70.

A procedure for determining the impurity concentration and the thicknessof each semiconductor layer (the semiconductor substrate 20, the driftregion 30, and the body region 40) of the main element domain 1 and theimpurity concentration and the thickness of each semiconductor layer(the semiconductor substrate 20, the drift region 30, and the bodyregion 40) of the sense element domain 2 will be described in detaillater.

Next, an operation of the semiconductor device will be described. Inorder to activate the semiconductor device, an ON potential (i.e., apotential equal to or higher than a minimum potential (gate thresholdpotential) at which a channel is formed in the body region 40) isapplied to each trench gate electrode 50 of each of the MOSFETs of themain element domain 1 and the MOSFETs of the sense element domain 2.When the ON potential is applied to the respective trench gate electrode50, the channel is formed in the body region 40 near the insulating film51. This permits electrons to flow from the source region 61 to thedrain electrode 10 through the channel formed in the body region 40, thedrift region 30, and the semiconductor substrate 20. In other words, inthe main element domain 1, a main current I_(m) flows from the drainelectrode 10 to the main source electrode 81. In the sense elementdomain 2, a sense current I_(s) flows from the drain electrode 10 to thesense source electrode 82. Since the source region 61 is not formed inthe insulating region 83, no current flows through the insulating region83.

FIG. 4 is a schematic diagram showing a flow of the main current I_(m)which flows through the main element domain 1 and a flow of the sensecurrent i which flows through the sense element domain 2. As shown inFIG. 4, a large number of MOSFETs (D_(M1) to D_(M4)) is disposed in themain element domain 1, and a small number of MOSFETs (D_(s)) is disposedin the sense element domain 2.

In the main element domain 1, currents I_(M1), I_(M2), I_(M3), andI_(M4) flow through the MOSFETs (D_(M1) to D_(M4)), respectively. Themain current I_(m) flowing through the MOSFETs (D_(M1) to D_(M4)) flowsto a load (for example, a motor) through the main source electrode 81and the source pad 85. The potential of the main source electrode 81 isinput to the current detection circuit through the Kelvin pad 86 and awiring line.

In the sense element domain 2, the sense current I_(s) flows through theMOSFET (D_(s)). The sense current I_(s) flowing through the MOSFET(D_(s)) is input to the current detection circuit through the sensesource electrode 82 and the sense source pad 84. The current detectioncircuit detects a current value of the sense current I_(s) received fromthe sense source pad 84. When the current value of the sense currentI_(s) is detected, the value of the main current I_(m) flowing throughthe main element domain 1 is calculated based on the detected currentvalue of the sense current I_(s) and the current sense ratio.

Next, the procedure for determining the impurity concentration and thethickness of each semiconductor layer (i.e., the semiconductor substrate20, the drift region 30, and the body region 40) of each of the mainelement domain 1 and the sense element domain 2 will be explained. FIG.5 is a flow chart showing the procedure for determining the impurityconcentration and the thickness of each semiconductor layer of each ofthe main element domain and the sense element domain.

As shown in FIG. 5, the impurity concentration and the thickness of eachsemiconductor layer (the semiconductor substrate 20, the drift region30, and the body region 40) of the main element domain 1 are temporarilydetermined, and the impurity concentration and the thickness of eachsemiconductor layer (the semiconductor substrate 20, the drift region30, and the body region 40) of the sense element domain 2 aretemporarily determined (step S10). As a specific example of thedetermination method, the impurity concentration and the thickness ofeach of the semiconductor layers 20, 30, and 40 of the main clementdomain 1 are determined depending on the intended use of thesemiconductor device. Next, the impurity concentration and the thicknessof each of the semiconductor layers 20, 30, and 40 of the sense elementdomain 2 may be determined based on the determined impurityconcentration and the thickness of each of the semiconductor layers 20,30, and 40 of the main element domain 1. In a first calculation, theimpurity concentration and the thickness of each of the semiconductorlayers 20, 30, and 40 of the sense element domain 2 may be set to valuesequal to those of the impurity concentration and the thickness of eachof the semiconductor layers 20, 30, and 40 of the main element domain 1.If a deviation of the evaluation value calculated in step S18, whichwill be described later, is not less than a predetermined value, theimpurity concentration and the thickness of each of the semiconductorlayers 20, 30, and 40 of the main element domain 1 and/or the senseelement domain 2 can be changed based on the calculation result

After the determination of the impurity concentration of each of thesemiconductor layers 20, 30, and 40 of each of the main element domain 1and the sense element domain 2, a temperature-dependent resistancechanging rate (R_(Mi), R_(Si), (i=20, 30, 40)) of each of thesemiconductor layers 20, 30, and 40 is calculated using the impurityconcentration (step S12). The term “temperature-dependent resistancechanging rate” herein described refers to a value for quantitativelyevaluating a degree of change of a resistance value with respect to atemperature change. Accordingly, not only a change rate of a resistancevalue itself, but also a change rate of a physical quantity (forexample, a current value) that changes based on a change in resistancevalue, for example, may be used. In this embodiment, a value obtained bydividing a current value obtained at a predetermined temperature (forexample, a maximum temperature within a temperature range used by thesemiconductor device) by a current value obtained at a referencetemperature (for example, 25° C.) is used as a temperature-dependentresistance changing rate.

In order to calculate the temperature-dependent resistance changingrate, a predetermined potential is first applied to a unit volume ofeach of the semiconductor layers 20, 30, and 40 at the referencetemperature (25° C.), and a value of current flowing through eachsemiconductor layer is obtained. Then, an appropriate temperature isselected from the temperature range (for example, 0 to 150° C.) used bythe semiconductor device, and the predetermined potential is applied tothe unit volume of each of the semiconductor layers 20, 30, and 40 atthe selected temperature, thereby obtaining the value of current flowingthrough each semiconductor layer. The current value obtained at theselected temperature is divided by the current value obtained at thereference temperature, to thereby calculate the rate of the currentvalues (temperature-dependent resistance changing rate). The currentvalue at each temperature may be obtained by experiments orcalculations. In the case of obtaining the current value bycalculations, a well-known device simulator (for example, TCAD(GENNESIS) manufactured by ISE corporation) may be used.

FIG. 6 shows values of currents respectively flowing through thesemiconductor layers 20, 30, and 40 when the predetermined potential isapplied to each of the semiconductor layers 20, 30, and 40. FIG. 6 showsa ratio of a current value at each temperature to a current value at thereference temperature (temperature-dependent resistance changing rate)with the current value at the reference temperature (25° C.) as thereference. As is obvious from FIG. 6, when the temperature of thesemiconductor device increases, the values of currents respectivelyflowing through the semiconductor substrate 20 (“Substrate” in FIG. 6),the drift layer 30 (“Drift” in FIG. 6), and the body region 40(“Channel” in FIG. 6) decrease. In other words, the resistance values ofthe semiconductor substrate 20, the drift layer 30, and the body region40 increase. Further, as is obvious from FIG. 6, thetemperature-dependent resistance changing rate of the drift layer 30 issubstantially equal to that of the body region 40, and thetemperature-dependent resistance changing rate of the semiconductorsubstrate 20 is smaller than that of the drift layer 30 and the bodyregion 40. The results shown in FIG. 6 were obtained under theconditions that the impurity concentration of the semiconductorsubstrate 20 is 23×e¹⁹ cm⁻³, the impurity concentration of the driftregion 30 is 2.0×e¹⁶ cm⁻³, and the impurity concentration of the bodyregion 40 is 1.0×e¹⁷ cm⁻³.

Next, the resistance ratio of each of the semiconductor layers 20, 30,and 40 of each of the main element domain 1 and the sense element domain2 is calculated (step S14). The term “resistance ratio” herein describedrefers to an amount (ratio) of resistance components included in each ofthe semiconductor layers 20, 30, and 40 relative to the entiresemiconductor region 100. As the resistance ratio, a resistance ratio ofeach of the semiconductor layers 20, 30, and 40 obtained at thereference temperature may be used.

As a specific method for calculating the resistance ratio, theresistance ratio may be calculated by a simulation using a well-knowndevice simulator (for example, TCAD (GENNESIS) manufactured by ISECorporation). That is, the potential or the like of each part in themain element domain 1 can be calculated by a simulation performed on themain element domain 1, and the resistance ratio of each of thesemiconductor layers 20, 30, and 40 of the main element domain 1 can becalculated from this calculation result. Further, the potential or thelike of each part in the sense element domain 2 can be calculated by asimulation performed on the sense element domain 2, and the resistanceratio of each of the semiconductor layers 20, 30, and 40 of the senseelement domain 2 can be calculated from this calculation result.

FIG. 8 shows a simulation model of the main element domain 1. As shownin FIG. 8, the simulation model of the main element domain 1 is obtainedby modeling only one MOSFET. This is because, in the main element domain1, the effect of current flowing from the outside of the main elementdomain is small and thus there is no need to take into consideration thecurrent flowing obliquely. The arrow in FIG. 8 indicates a currentflowing direction.

FIG. 9 shows a simulation model of the sense element domain 2. As shownin FIG. 9, the simulation model of the sense element domain 2 isobtained by modeling two MOSFETs and a non-element region (a region inwhich no MOSFET is formed) disposed adjacent to these MOSFETs. In thesense element domain 2, the effect of current flowing from the outsideof the sense element domain is large. Accordingly, it is necessary totake into consideration the current flowing obliquely (indicated by thearrow extending obliquely in FIG. 9). The number of MOSFETs incorporatedin the simulation model and the size of the non-element region can beappropriately determined based on the layout of the main element domain1 and the sense element domain 2.

FIG. 7 shows a result of calculating the resistance ratio of each of thesemiconductor layers 20, 30, and 40 by a simulation using the simulationmodels (FIGS. 8 and 9) described above. As is obvious from FIG. 7, inthe sense element domain 1, the resistance ratio of the semiconductorsubstrate 20 is small and the resistance ratio of each of the driftregion 30 and the body region 40 is large. On the other hand, in themain element domain 2, the resistance ratio of the semiconductorsubstrate 20 is larger than that of the sense element domain 1.

As described above, the temperature-dependent resistance changing rateof the semiconductor substrate 20 is greatly different from thetemperature-dependent resistance changing rate of each of the driftregion 30 and the body region 40 (see FIG. 6). In the main elementdomain 1, the resistance ratio of the semiconductor substrate 20 islarge, while in the sense element domain 2, the resistance ratio of thesemiconductor substrate 20 is small. Accordingly, when the temperatureof the semiconductor device changes, the difference between theresistance value of the main element domain 1 and the resistance valueof the sense clement domain 2 becomes large, resulting in a change incurrent sense ratio.

After the calculation of the resistance ratio, an evaluation value forevaluating the temperature-dependent resistance changing rate of each ofthe main element domain 1 and the sense element domain 2 is calculatedusing the temperature-dependent resistance changing rate calculated instep S12 and the resistance ratio calculated in step S14 (step S16).Specifically, the temperature-dependent resistance changing rate(R_(Mi)) of each of the semiconductor layers 20, 30, and 40 ismultiplied by the resistance ratio (k_(Mi)) of each of the semiconductorlayers 20, 30, and 40 for the main element domain 1, and the sum ofthese values is obtained. Similarly, the temperature-dependentresistance changing rate (R_(Si)) of each of the semiconductor layers20, 30, and 40 is multiplied by the resistance ratio (k_(Si)) of each ofthe semiconductor layers 20, 30, and 40 for the sense element domain 2,and the sum of these values is obtained.

Next, an index value for evaluating a temperature-dependent variation ofa current sense ratio is calculated from the evaluation valueΣ(R_(Mi)×k_(Mi)) of the main element domain 1 and the evaluation valueΣ(R_(Si)×k_(Si)) of the sense element domain 2, which are calculated instep S16, to determine whether the index value is smaller than apredetermined value (step S18). Specifically, the evaluation valueΣ(R_(Si)×k_(Si)) of the sense element domain 2 is subtracted from theevaluation value Σ(R_(Mi)×k_(Mi)) of the main element domain 1, and thesubtracted value is divided by the evaluation value Σ(R_(Mi)×k_(Mi)) ofthe main element domain 1.

When the calculated index value is smaller than the predetermined value,it can be determined that the temperature-dependent variation of thecurrent sense ratio falls within a desired range. Accordingly, theimpurity concentration and the thickness of each of the semiconductorlayers 20, 30, and 40, which are temporarily determined in step S10, aredetermined as the final impurity concentration and thickness. When theindex value exceeds the predetermined value, it can be determined thatthe temperature-dependent variation of the current sense ratio exceedsthe desired range. Therefore, the flow returns to step S10, and theprocessing from step S10 is repeated. Specifically, the impurityconcentration and the thickness of each of the semiconductor layers 20,30, and 40 of the main element domain 1 and/or the sense element domainare changed so that the index value is set to be smaller than thepredetermined value. Thus, conditions for setting the index value to besmaller than the predetermined value can be determined.

Referring now to FIGS. 10 and 11, a specific example of calculating theindex value in step S18 will be described. In this calculation example,assume that the impurity concentration of the semiconductor substrate 20is 23×e¹⁹ cm⁻³; the impurity concentration of the drift region 30 is2.0×e¹⁶ cm⁻³; and the impurity concentration of the body region 40 is1.0×e¹⁷ cm⁻³.

Referring to FIG. 10, a current ratio (i.e., a temperature-dependentresistance changing rate) of each of the semiconductor layers 20, 30,and 40 is calculated at each of 100° C. and 150° C. Referring to FIG.11, the resistance ratio of each of the semiconductor layers 20, 30, and40 of the main element domain 1 and the resistance ratio of each of thesemiconductor layers 20, 30, and 40 of the sense element domain 2 at thereference temperature (25° C.) are calculated.

The index value is calculated in the following manner using thetemperature-dependent resistance changing rate obtained at 150° C. Asshown in FIG. 10, the temperature-dependent resistance changing rate ofthe semiconductor substrate 20 at 150° C. is 0.78; thetemperature-dependent resistance changing rate of the drift region 30 is0.52; and the temperature-dependent resistance changing rate of the bodyregion 40 is 0.43. Accordingly, the evaluation value for evaluating thetemperature-dependent resistance changing rate of the main elementdomain 1 is calculated by 0.78×0.30+0.52×0.36+0.43×0.34=0.567. Theevaluation value for evaluating the temperature-dependent resistancechanging rate of the sense element domain 2 is calculated by0.78×0.03+0.52×0.43+0.43×0.54=0.479. Thus, the index value forevaluating the temperature-dependent variation of the current senseratio is calculated by (0.567−0.479)×0.567=0.155. As a result, thetemperature-dependent variation of the current sense ratio can beevaluated as 15.5%.

Here, an exemplary configuration of the semiconductor device capable ofreducing the index value (temperature-dependent variation of currentsense ratio) will be described.

As shown in FIG. 11, the resistance ratio of the body region (i.e., theresistance ratio of the channel) in the sense element domain 2 is largerthan that of the main element domain 1. Accordingly, the above-mentionedindex value can be reduced by reducing the resistance ratio of the bodyregion of the sense element domain 2. For example, as shown in FIG. 15,the thickness of the body region 44 of the sense element domain 2 is setto be smaller than that of the body region 42 of the main element domain1. This results in a reduction in the resistance ratio of the bodyregion of the sense element domain 2 and a reduction in the index value(temperature-dependent variation of current sense ratio).

The impurity concentration of the body region of the sense elementdomain 2 may be reduced without changing the thickness of the bodyregion 44 of the sense element domain 2. Also in this case, a reductionin the resistance ratio of the body region of the sense element domain 2and a reduction in the temperature-dependent variation of the currentsense ratio can be achieved.

As shown in FIG. 11, the resistance ratio of the drift region of thesense element domain 2 is larger than that of the main element domain 1.Accordingly, the index value can be reduced by reducing the resistanceratio of the drift region of the sense element domain 2. For example, asshown in FIG. 16, the thickness of the drift region 34 of the senseelement domain 2 is set to be smaller than that of the drift region 32of the main element domain 1. This results in a reduction in theresistance ratio of the drift region of the sense element domain 2 and areduction in the temperature-dependent variation of the current senseratio.

Also in this case, the resistance ratio of the drift region of the senseelement domain 2 may be reduced by changing the impurity concentrationof the drift region of the sense element domain 2 without changing thethickness of the drift region of the sense element domain 2.

Further, as shown in FIG. 11, the resistance ratio of the semiconductorsubstrate in the sense element domain 2 is smaller than that of the mainelement domain 1. Accordingly, the index value can be reduced byincreasing the resistance ratio of the semiconductor substrate in thesense element domain 2. For example, as shown in FIG. 17, the thicknessof a semiconductor substrate 24 in the sense element domain 2 is set tobe larger than that of a semiconductor substrate 22 in the main elementdomain L This results in an increase in the resistance ratio of thesemiconductor substrate 24 in the sense element domain 2 and a reductionin the temperature-dependent variation of the current sense ratio.

Also in this case, the resistance ratio of the semiconductor substratein the sense element domain 2 may be increased by changing the impurityconcentration of the semiconductor substrate without changing thethickness of the semiconductor substrate in the sense element domain 2.

The predetermined value to be compared in step S18 with the index value(index value for evaluating the temperature-dependent variation of thecurrent sense ratio) can be appropriately determined depending on theperformance required for the semiconductor device. For example, assumingthat the predetermined value is 0.1 (assuming that the difference is 10%or less), the variation of the current sense ratio within thetemperature range of 25° C. to 150° C. can be set to 15% or less.Assuming that the predetermined value is 0.07 (assuming that thedifference is 7% or less), the variation of the current sense ratiowithin the temperature range of 25° C. to 150° C. can be set to 6% orless. Assuming that the predetermined value is 0.05 (assuming that thedifference is 5% or less), the variation of the current sense ratiowithin the temperature range of 25° C. to 150° C. can be set to 5% orless. Assuming that the predetermined value is 0.02 (assuming that thedifference is 2% or less), the variation of the current sense ratiowithin the temperature range of 25° C. to 150° C. can be set to 3% orless. Thus, the predetermined value may be set depending on thevariation of the temperature-dependent sense current ratio required forthe semiconductor device.

As is obvious from the above description, in the semiconductor device ofthis embodiment, the evaluation value Σ(R_(Mi)×k_(Mi)) for evaluatingthe temperature-dependent resistance changing rate of the entire mainelement domain 1 and the evaluation value Σ(R_(Si)×k_(Si)) forevaluating the temperature-dependent resistance changing rate of theentire sense element domain 2 are calculated using thetemperature-dependent resistance changing rate and the resistance ratioof each of the semiconductor layers 20, 30, and 40. In the case ofcalculating the resistance ratio of the sense element domain 2, senseelements (MOSFETs) as well as the non-element region adjacent to thesense elements are incorporated in the simulation models. As a result,the resistance ratio of each of the semiconductor layers 20, 30, and 40can be calculated in consideration of the current flowing obliquelythrough the semiconductor layers 20, 30, and 40. Accordingly, theevaluation value Σ(R_(Si)×k_(Si)) for evaluating thetemperature-dependent resistance changing rate of the sense elementdomain 2 described above is a value obtained by taking intoconsideration the current flowing obliquely through the semiconductorregion 100. Then, based on the value obtained by taking intoconsideration the current flowing obliquely through the semiconductorregion 100, the impurity concentration and the thickness of each of thesemiconductor layers 20, 30, and 40 are changed so that the differencebetween the temperature-dependent resistance changing rate of the entiremain element domain 1 and the temperature-dependent resistance changingrate of the entire sense element domain 2 can be reduced. Consequently,a semiconductor device having a small temperature-dependent variation ofthe current sense ratio can be obtained.

The semiconductor device of this embodiment is capable of reducing thetemperature-dependent variation of the current sense ratio, therebyeliminating the need to design another component assuming that thetemperature-dependent variation of the current sense ratio deteriorates.In other words, if the temperature-dependent variation of the currentsense ratio is large, the accuracy of the calculated main current valueis lowered. This raises a need to use a component for large current as afuse for forcibly blocking current flowing to the semiconductor device,or as a wire harness connected to the semiconductor device. In thesemiconductor device of this embodiment, however, thetemperature-dependent variation of the current sense ratio is small, andthus the accuracy of the calculated main current value is increased. Asa result, a component for low current can be used as a fuse or a wireharness, which leads to a reduction in manufacturing costs.

The semiconductor device of this embodiment can be suitably used for acontrol circuit for controlling charging/discharging of a secondary cellsuch as a lithium cell. In order to accurately charge/discharge thesecondary cell, it is necessary to accurately detect a value of currentflowing to the secondary cell and a value of current flowing out of thesecondary cell. Meanwhile, in the case of charging/discharging thesecondary cell, the secondary cell generates heat, so that thetemperature of the control circuit varies. The semiconductor device ofthis embodiment has a small temperature-dependent variation of thecurrent sense ratio. Accordingly, even when the temperature of thecontrol circuit changes during charging/discharging of the secondarycell, a value of a charging current flowing to the secondary cell and avalue of a discharging current flowing out of the secondary cell can bedetected accurately. This makes it possible to charge/discharge thesecondary cell with high accuracy.

Furthermore, since the semiconductor device of this embodiment has asmall temperature-dependent variation of the current sense ratio, thesemiconductor device can be used for various purposes (for example, acompact DCDC converter), unlike the related art. This eliminates theneed for a shunt resistor which has been conventionally needed. Theshunt resistor has a particularly large calorific value, which requiresa countermeasure for the heat and results in a complicated thermaldesign. Therefore, the advantageous effect of eliminating the need forthe shunt resistance is large.

In the embodiment described above, a structure such as a trench is notformed in the insulating region 83 between the main element domain 1 andthe sense element domain 2. However, as shown in FIGS. 12 to 14,trenches 114, 116, and 118 may be formed in the region between the mainelement domain 1 and the sense element domain 1 The trenches 114, 116,and 118 are preferably formed to surround the sense element domain 2. Anoxide film, a polysilicon layer, or an n⁺ layer may be filled in each ofthe trenches 114, 116, and 118. In any case, the trenches 114, 116, and118 are formed so as to prevent current from flowing across thetrenches. The depth of each of the trenches 114, 116, and 118 can bearbitrarily determined. For example, as shown in FIG. 12, the trench 114may be formed in the range from the body region 102 to the drift region104. Alternatively, as shown in FIG. 13, the trench 116 may be formedonly within the range of the body region 102. More alternatively, asshown in FIG. 14, the trench 118 may be formed in the range includingthe body region 102, the drift region 104, and the semiconductorsubstrate 104. The range of the region in which current is controlled tobe prevented from flowing obliquely can be selected by changing thedepth of each of the trenches 114, 116, and 118.

The formation of the trenches 114, 116, and 118 as described aboveprevents current from flowing obliquely through the semiconductorregion. Particularly in the sense element domain 2, current is preventedfrom flowing from the outside of the sense element domain 2. As aresult, the resistance ratio of each of the semiconductor layers 20, 30,and 40 in the main element domain 1 is close to that in the senseelement domain 2, so that the temperature-dependent variation of thecurrent sense ratio can be reduced.

Although the above embodiment illustrates an example in which thepresent invention is applied to MOSFETs, the present invention can alsobe applied to other semiconductor devices such as an IGBT and a diode.Furthermore, the above embodiment illustrates an example in which thepresent invention is applied to a vertical semiconductor device, but thepresent invention can also be applied to a lateral semiconductor device.

The present invention also provides a design support device thatexecutes the processing shown in FIG. 5. Such a design support devicemay include a storage device for storing a program for executing theprocessing shown in FIG. 5; an arithmetic unit for executing the programstored in the storage device; an input device for inputting calculationconditions and a simulation model to the arithmetic unit; and an outputdevice (for example, a display device) for outputting calculationresults from the arithmetic unit. When a designer inputs designconditions (such as an impurity concentration and a thickness of eachsemiconductor layer) to the design support device, the design supportdevice calculates an index value, and outputs the calculated index valueto the output device. This allows the designer to easily determine anappropriate impurity concentration and the thickness of eachsemiconductor layer.

Specific embodiment of the present invention is described above, butthis merely illustrates some representative possibilities for utilizingthe invention and does not restrict the claims thereof The subjectmatter set forth in the claims includes variations and modifications ofthe specific examples set forth above.

The technical elements disclosed in the specification or the drawingsmay be utilized separately or in all types of combinations, and are notlimited to the combinations set forth in the claims at the time offiling of the application. Furthermore, the subject matter disclosedherein may be utilized to simultaneously achieve a plurality of objectsor to only achieve one object.

1.-7. (canceled)
 8. A semiconductor device comprising: a main elementdomain; and a sense element domain disposed adjacent to the main elementdomain; wherein each of the main element domain and the sense elementdomain comprises a first semiconductor layer, a second semiconductorlayer laminated on the first semiconductor layer, and a thirdsemiconductor layer laminated on the second semiconductor layer, and animpurity concentration of the second semiconductor layer is lower thanthat of the first semiconductor layer, wherein when the semiconductordevice turns on, a channel is formed in each of the third semiconductorlayers of the main element domain and the sense element domain, andcurrent flows from the second semiconductor layer to the thirdsemiconductor layer or from the third semiconductor layer to the secondsemiconductor layer, and at least an impurity concentration of the thirdsemiconductor layer of the sense element domain is lower than that ofthe third semiconductor layer of the main element domain.
 9. Thesemiconductor device as in claim 8, wherein in at least one of thesemiconductor layers from the first semiconductor layer to the thirdsemiconductor layer of the main element domain and the sense elementdomain, at least one of an impurity concentration and a thickness of theat least one of the semiconductor layers of the main element domain isdifferent from that of the at least one of the semiconductor layers ofthe sense element domain such that a value calculated by a followingequation is less than a predetermined value:$\left\lbrack {{\sum\limits_{i = 1}^{n}\left( {R_{Mi} \times k_{Mi}} \right)} - {\sum\limits_{i = 1}^{n}\left( {R_{Si} \times k_{Si}} \right)}} \right\rbrack/{\sum\limits_{i = 1}^{n}\left( {R_{Mi} \times k_{Mi}} \right)}$where a temperature-dependent resistance changing rate of an i-thsemiconductor layer (i=1 to 3) of the main element domain is R_(Mi), aresistance ratio of the i-th semiconductor layer of the main elementdomain relative to the entire main element domain is k_(Mi), atemperature-dependent resistance changing rate of the i-th semiconductorlayer of the sense element domain is R_(Si), and a resistance ratio ofthe i-th semiconductor layer of the sense element domain to the entiresense element domain is k_(Si).
 10. The semiconductor device as in claim9, wherein a thickness of the third semiconductor layer of the senseelement domain is smaller than that of the third semiconductor layer ofthe main element domain.
 11. The semiconductor device as in claim 10,wherein a thickness of the first semiconductor layer of the senseelement domain is greater than that of the first semiconductor layer ofthe main element domain.
 12. The semiconductor device as in claim 11,wherein a thickness of the second semiconductor layer of the senseelement domain is smaller than that of the second semiconductor layer ofthe main element domain.
 13. The semiconductor device as in claim 12,wherein a trench is formed between the main element domain and the senseelement domain.
 14. The semiconductor device as in claim 8, wherein athickness of the third semiconductor layer of the sense element domainis smaller than that of the third semiconductor layer of the mainelement domain.
 15. The semiconductor device as in claim 8, wherein athickness of the second semiconductor layer of the sense element domainis smaller than that of the second semiconductor layer of the mainelement domain.